FLASH radiotherapy (FLASH-RT), with the delivery of ionizing radiation at ultra-high dose rate, typically ≥ 40 Gy/s, has emerged as one of the most debated and potentially transformative developments in modern radiation oncology. Unlike conventional radiotherapy, which optimizes spatial dose distribution, FLASH-RT alters the temporal structure of dose delivery, thereby establishing time as a dominant radiobiological variable. Despite intense global interest, the biological basis of the FLASH effect remains unresolved, reproducibility varies across experimental conditions, and clinical translation remains limited to specialized systems. This review differs from prior summaries by explicitly separating well-established biological effects from speculative mechanistic interpretations and early clinical feasibility. It does not seek to catalog all published FLASH radiotherapy studies, but rather to critically synthesize the literature by highlighting where consensus exists, where interpretations diverge, and where caution is warranted in clinical extrapolation. It aims to prevent overinterpretation of FLASH-RT’s current translational readiness. A structured comparison with conventional radiotherapy is presented, along with a discussion of unresolved controversies, practical limitations, and realistic future directions.